1. Field of the Invention
The present invention relates to ceramic adhesive compositions which are useful for refractory applications.
2. Background Information
Dolomite, particularly calcined dolomite (doloma), is used to make various types of refractory articles for use in a high-temperature environment such as an environment encountered in metallurgical applications. It is often desirable to secure refractory articles together. In addition, it is sometimes necessary to fill cracks, spaces, gaps and the like in a shaped refractory article and it is also sometimes necessary to patch defects which develop in refractory articles.
Refractory articles may be mechanically joined together. However, such mechanical devices for joining refractory articles are usually cumbersome to use and may not be suitable for use in a high-temperature environment. Clearly, it is desirable to provide a suitable adhesive which can be used to secure shaped refractory articles together, particularly articles made from doloma. Such an adhesive must be compatible with the doloma and should have similar physical properties, especially refractory properties. Thus, it is highly desirable to provide refractory cement which contains calcined dolomite, magnesia (MgO) or mixtures of MgO and CaO. Refractory adhesives which contain the aforementioned ingredients must be formulated with a binder system so that the adhesive composition has a sufficient viscosity for application to the surfaces being joined or for application to fill cracks, gaps or spaces or for use as a patch to repair defects. In addition, such a binder should be capable of quickly setting for the formation of a bond having adequate green strength to keep the refractory articles together. Also, the cement should develop a strong ceramic matrix bond upon firing so that the objects become permanently bonded.
At present, ceramic based refractory articles use aqueous mortar compositions with alkali silicate and alkali phosphate binders. However, the hydratable nature of calcined dolomite, MgO and combinations of MgO and CaO makes these materials unsuitable for use with a binder containing water.
Non-aqueous binder systems are known for use with articles made out of doloma or magnesia and related refractories. However, these articles, such as bricks, refractory casting molds and the like are not formulated for use as an adhesive or fillet composition. In particular, these types of compositions which are used for making bricks, casting molds, etc., are required to have certain physical characteristics which make them unsuitable for use as an adhesive or filler. For example, bricks are desirably made with coarse particles size refractory grain and an amount of binder so that the composition can be compacted to form a fixed shape which is then fired to form the finished brick. In contrast, an adhesive composition is required to be in the form of a paste. When formulated into a paste, the adhesive composition may be packed in a caulking cartridge and extruded by means of a conventional caulking gun or may be trowelled or parged onto an existing refractory shaped article. Likewise, use of the composition as a filler requires the same type of paste consistency.
In addition to the above-discussed characteristics, a refractory adhesive composition should be air-dryable, i.e., the binder should harden or set upon exposure to air so that sufficient green strength develops for adhesively securing refractory articles together prior to firing.